Effects of Framework Disruption of Ga and Ba Containing Zeolitic

catalysts

Article Eﬀects of Framework Disruption of Ga and Ba Containing Zeolitic Materials by Thermal Treatment

Siyabonga S. Ndlela 1 , Holger B. Friedrich 1,* and Mduduzi N. Cele 2

1 School of Chemistry and Physics, Westville Campus, University of KwaZulu–Natal, Private Bag X 54001, Durban 4000, South Africa; [email protected] 2 School of Physical and Chemical Sciences, Maﬁkeng Campus, North-West University, Private Bag X 2046, Mmabatho 2745, South Africa; [email protected] * Correspondence: [email protected]; Tel.: +27-31-2603-107

Received: 21 July 2020; Accepted: 29 July 2020; Published: 30 August 2020

Abstract: The effect of the thermal treatment of some zeolitic materials was studied on oxidative dehydrogenation (ODH) of n-octane. Gallium containing faujasite catalysts were synthesized using isomorphic substitution, specifically, a galosilicalite (Ga-BaY(Sil)) and an aluminosilicalite substituted with gallium (Ga-BaY(IS)), with constant Si/M ratio. The catalysts were thermally treated at different temperatures (250, 550, and 750 ◦C) before catalytic testing. The quantification of total and strength of acid sites by FT-IR (O-H region), pyridine-IR, and NH3-temperature-programmed desorption (TPD) confirmed a decrease in the number of Brønsted acid sites and an increase in the number of Lewis acid sites upon increasing the calcination temperature. Isothermal n-octane conversion also decreased with the catalysts’ calcination temperature, whereas octene selectivity showed the opposite trend (also at iso-conversion). The COx selectivity showed a decrease over the catalysts calcined from 250 to 550 ◦C and then an increase over the 750 ◦C calcined catalysts, which was due to the strong adsorption of products to strong Lewis acid sites on the catalysts leading to the deep oxidation of the products. Only olefinic-cracked products were observed over the 750 ◦C calcined catalysts. This suggested that the thermal treatment increases Lewis acid sites, which activate n-octane using a bimolecular mechanism, instead of a monomolecular mechanism.

Keywords: oxidative dehydrogenation; n-octane; faujasite; zeolite; silicalite; isomorphic substitution

1. Introduction Zeolite materials offer a variety of properties in catalysis, resulting from their ability to be easily tuned both during synthesis and post synthesis. The introduction of other multivalent metals such as Fe, Ga, V, and Ti in the framework of zeolites can be carried out by substitution of aluminum during synthesis by an isomorphic substitution method [1,2]. Other cations, such as charge balancing cations on the extra framework of the zeolite (Na, K, Ba, etc.) can be introduced after synthesis by ionic exchange and impregnation [2]. Zeolites, however, are not ideal catalysts in the oxidative dehydrogenation of alkanes because of their highly acidic nature. The acidity of zeolites leads to unwanted side reactions that are selective to cracked products, catalyst deactivation as a result of coke deposition, and high COx production [3,4]. Therefore, it is of necessity to modify the acidic properties of zeolites to benefit the oxidative dehydrogenation (ODH) reactions in order to exploit other good properties (high surface area, thermal stability, high crystallinity, well-defined pores) of zeolites [1,2]. The introduction of group two alkaline earth metals by ionic exchange has shown to decrease the strength and overall acidity of the zeolite [5]. The introduction of other multivalent metals in the framework of a zeolite other than aluminum has also shown to decrease the acidity of zeolites [6].

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These modifications of zeolites have led to the synthesis of aluminum-free materials called silicalites, (Scheme S1) which have less acidity compared to the unmodified aluminosilicalite (zeolite). Zeolites and their analogues contain both Brønsted acid sites and Lewis acid sites, and these sites play different roles in the ODH of alkanes. Depending on the concentration of each acid site, the zeolite can either be a Brønsted acid or Lewis acid zeolite. Brønsted acid sites are associated with the hydroxyl groups bridging between the Si(OH)T groups of the zeolitic material, (T = multivalent metal), whereas Lewis acid sites are associated with the metallic species of the framework and extra framework of the material [7]. For hydrocarbon reactions, the Brønsted acid sites are known to directly activate the hydrocarbon by protonation of the feed molecule, whereas Lewis acid sites are known to induce a strong electrostatic field that can polarize adsorbed molecules which results in their activation [8]. Zeolitic materials treated at temperatures below 450 ◦C are mostly Brønsted acidic and prevalently associated with the Si(OH)T sites. Thermally treating the zeolitic materials causes the T atoms to migrate into partial or total extra framework positions (Scheme S2). This is true for samples treated at temperatures around 650 ◦C and for samples with the lowest Si/M ratios, such as faujasites [9]. Partial migration of T atoms means that the framework has been disrupted, but the atoms are still linked to the framework in a form of low coordinated and isolated species. These species at lattice defects are of strong Lewis character. The final step of this migration ends with the complete breaking of the bonds linking the T atoms to the framework, leading to the free migration of T atoms to Lewis acid centers and the formation of a silanol nest [9]. In ODH reactions, it is important, therefore, to understand the migration of the zeolitic material’s acid sites, as this influences the cracking mechanism, as well as the overall feed molecule activation mechanism. It is known that in alkane cracking on zeolitic materials, there is an involvement of carbonium ions as well as carbenium ions which leads to cracking via a ß-scission. The formation of the two species leads to two mechanisms; namely, monomolecular and bimolecular mechanisms for Brønsted acid sites and Lewis acid sites, respectively [10,11]. The monomolecular mechanism is the more pronounced and faster of the two mechanisms, it proceeds via the feed molecule protonation, resulting in a pentacoordinated carbonium ion which can crack to give off a smaller alkane fragment and an adsorbed carbenium ion. This ion subsequently cracks by ß-scission resulting in an olefin and a smaller carbenium ion. Lewis acid sites facilitated bimolecular mechanisms proceed via a hydride ion abstraction by a Lewis acid site from the feed molecule to form a carbenium ion which then cracks via a repeated ß-scission [10,11]. Therefore, by tracking the reaction product selectivity and conversion, one can probe the dominating mechanism in the ODH of n-octane when using thermally modified zeolitic materials. n-Octane was chosen as the representative medium to long chain alkane, which are very abundant and of low value. The ODH reaction was studied because it offers an interesting and less energy-intensive route for the production of olefins from paraffins by rendering the reaction exothermic as a result of the insertion of an oxygen source into the reaction, thus suppressing the thermodynamic constraints [12].

2. Results and Discussion

2.1. Catalysts Characterization

2.1.1. Powder XRD The XRD pattern of both the prepared catalysts (Figure S1) showed the already known crystal structure of the faujusite type zeolite with characteristic reflections centered around 21.8◦, 26.4◦, and 54.5◦ [13]. There were no diffraction peaks observed for any gallium oxide species which suggests good dispersion of the metal within the zeolite framework. Complete substitution of aluminum with gallium in the zeolite framework did not affect the crystal structure of the zeolite, as both the aluminosilicalite and galosilicalite catalysts showed similar diffractograms. A slight shifting of peaks was observed which could be attributed to the difference in the size of aluminum and gallium in the zeolitic framework. Catalysts 2020, 10, 975 3 of 13 Catalysts 2020, 10, x FOR PEER REVIEW 3 of 13 was2.1.2. observed FT-IR which could be attributed to the difference in the size of aluminum and gallium in the zeolitic framework. The IR assignments for the two prepared catalysts are similar to previously reported faujasite 2.1.2.type FT-IR zeolitic materials (Figure S2) [12]. The shifts of the IR bands observed between the two spectra obtained can be attributed to change in lattice parameters induced by the substitution of aluminum The IR assignments for the two prepared catalysts are similar to previously1 reported faujasite typewith zeolitic gallium. materials The IR(Figure spectra S2) showed[12]. The bandsshifts of just the aboveIR bands 1800 observed cm− which between are the assigned two spectra to asymmetric 1 obtainedstretching can ofbe theattributed internal to tetrahedra,change in lattice and pa symmetricrameters induced stretching by the was substitution shown by of bands aluminum at 740–880 cm− . withThese gallium. bands The are IR weaker spectra for showed Ga-BaY(IS), bands just and above that could1800 cm be−1 attributed which are toassigned the di fftoerence asymmetric in crystal growth stretchingin the presence of the internal of aluminum tetrahedra, compared and symme to thetric stretching aluminum-free was shown Ga-BaY(Sil) by bands [ 14at ].740–880 The first cm− band1. around 1 These420 cmbands− is are indicative weaker for of theGa-BaY(IS), T-O bend, and and that the co externaluld be attributed linkage doubleto the difference ring was in shown crystal by the bands 1 1 growtharound in the 600 presence cm− . There of aluminum is a decrease compared in intensityto the aluminum-free of a ~3600 Ga-BaY(Sil) cm− band [14]. for The Ga-BaY(Sil), first band which is a aroundband 420 associated cm−1 is indicative with the of hydroxyl the T-O bend, group and attached the external to extra linkage framework double ring aluminum was shown [15 by– 17the]. −1 −1 bands aroundTo evaluate 600 cm the. There effect is a of decrease thermal in treatment,intensity of a the ~3600 catalyst cm band batches for Ga-BaY(Sil), calcined atwhich three different is a band associated with the hydroxyl group attached to extra framework aluminum [15–17]. temperatures (250, 500, and 750 C) were characterized by FT-IR in the O-H region (~3200–4200 cm 1). To evaluate the effect of thermal◦ treatment, the catalyst batches calcined at three different − temperaturesThis can give (250, information 500, and 750 on°C) thewere strength characterized and qualitativeby FT-IR in the amount O-H region of Brønsted (~3200–4200 acid cm sites.−1). Both the Thiscatalysts can give (Ga-BaY(IS) information and on Ga-BaY(Sil)) the strength showedand qualitative stronger amount absorbance of Brønsted bands acid (Figures sites.1 Both and 2the) for catalysts catalystscalcined (Ga-BaY(IS) at 250 ◦C, and and Ga-BaY(Sil)) the bands decreasedshowed strong wither increasing absorbance calcination bands (Figures temperature. 1,2) for catalysts There was also a 1 calcinedshift observed at 250 °C, inand the the IR bands bands, decreased with a samplewith increasing calcined calcination at 750 ◦C temperature. showing bands There closerwas also to 3800 cm− . a Thisshift observed may be attributedin the IR bands, to the with diff aerence samplein calcined the strength at 750 °C of showing the OH bands bonds, closer with to the 3800 strongest cm−1. shifting Thisto highermay be IRattributed regions to [18 the]. difference The disappearance in the strength of the of the OH OH bands bonds, with with increasing the strongest temperature shifting suggests tothat higher the IR number regions of[18]. Brønsted The disappearance acid sites of decreases the OH bands during with thermal increasing treatment temperature of the suggests catalysts through thatdehydroxylation the number of Brønsted of the zeolites acid sites [9 ].decreases The IR bandsduring forthermal the galosilicalite treatment of the (Ga-BaY(Sil) catalysts through were less intense dehydroxylation of the zeolites [9]. The IR bands for the galosilicalite (Ga-BaY(Sil) were less intense than for the aluminosilicalite (Ga-BaY(IS)), suggesting low Brønsted acid sites concentration for the than for the aluminosilicalite (Ga-BaY(IS)), suggesting low Brønsted acid sites concentration for the aluminum-freealuminum-free catalyst. catalyst.

0.55 A 0.52

0.49

0.46

0.43 B 0.40 Absorbance

0.37

0.34

0.31

0.28

0.25 C 0.22

4200 4000 3800 3600 3400 3200

Wavenumber (cm−1)

FigureFigure 1. FT-IR 1. FT-IR spectra spectra of the of O-H the region O-H region of Ga-BaY(IS) of Ga-BaY(IS) calcined calcinedat (A) 250 at °C, (A (B)) 250 550◦ °C,C, ((BC)) 550750 °C.◦C, (C) 750 ◦C.

Catalysts 2020, 10, x FOR PEER REVIEW 4 of 13

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Catalysts0.60 2020, 10, x FOR PEER REVIEW A 4 of 13

0.56 0.60 A

0.52 0.56

0.52 0.48

0.48 0.44 0.44 B 0.40 B 0.40 Absorbance

0.36Absorbance 0.36

0.32 0.32

0.28 0.28 C C

4200 4000 3800 3600 3400 3200 4200 4000 3800 3600 3400 3200 Wavenumber (cm−1) −1 Wavenumber (cm ) FigureFigure 2. FT-IR2. FT-IR spectra spectra of of thethe O–H region region of of Ga-BaY(Sil) Ga-BaY(Sil) calcined calcined at (A at) 250 (A )°C, 250 (B◦) C,550 ( B°C,) 550(C) 750◦C, °C. (C ) 750 ◦C. Figure 2. FT-IR spectra of the O–H region of Ga-BaY(Sil) calcined at (A) 250 °C, (B) 550 °C, (C) 750 °C. 2.1.3.2.1.3. ScanningScanning Electron Electron Microscopy Microscopy 2.1.3. ScanningFigureFigure3 3Electron shows the theMicroscopy cubic cubic surface surface morphology morphology of the prepared of the catalysts prepared which catalysts is the typical which morphology is the typical morphologyforFigure the faujasite 3 shows for zeolitic the the cubic faujasite materials. surface zeoliticThe morphology difference materials. in ofthe the crystal The prepared disizefference between catalysts in the the whichtwo crystal catalysts is the size typicalresults between from morphology the the different framework composition (aluminum and aluminum-free for Ga-BaY(IS) and Ga-BaY(Sil), for twothe faujasite catalysts zeolitic results materials. from the di Thefferent difference framework in the composition crystal size between (aluminum the andtwo aluminum-freecatalysts results for from respectively) which leads to different crystal growth rates during the synthesis [19,20]. theGa-BaY(IS) different framework and Ga-BaY(Sil), composition respectively) (aluminum which an leadsd aluminum-free to different crystal for Ga-BaY(IS) growth rates and during Ga-BaY(Sil), the respectively)synthesis [which19,20]. leads to different crystal growth rates during the synthesis [19,20].

(A) (B)

Figure 3. SEM micrographs of (A) Ga-BaY(Sil), (B) Ga-BaY(IS).

2.1.4. Pyridine FT-IR (A) (B) Pyridine IR was carried out at room temperature. Figure 4 and Figure 5 show the pyridine vibration range (1400–1700 cm−1) Figure[21]. For 3. both SEM the micrographs catalysts, two of (Abands) Ga-BaY(Sil), related to (Lewis-bondedB) Ga-BaY(IS). pyridine appeared at 1450 and 1624 cm−1. Pyridine physically adsorbed via hydrogen bonding with surface hydroxyl groups 2.1.4. Pyridine FT-IR 2.1.4. Pyridine FT-IR Pyridine IR was carried out at room temperature. Figures4 and5 show the pyridine vibration Pyridine IR was carried out at room temperature. Figure 4 and Figure 5 show the pyridine vibration range (1400–1700 cm 1)[21]. For both the catalysts, two bands related to Lewis-bonded pyridine range (1400–1700 cm−1) [21].− For both the catalysts, two bands related to Lewis-bonded pyridine appeared appeared at 1450 and 1624 cm 1. Pyridine physically adsorbed via hydrogen bonding with surface at 1450 and 1624 cm−1. Pyridine physically− adsorbed via hydrogen bonding with surface hydroxyl groups 1 hydroxyl groups and gave absorption bands at 1574 and 1595 cm− , which conﬁrmed the presence of 1 weak and medium Brønsted acid sites. A band at 1489 cm− was due to the contribution of both Lewis and Brønsted acid sites [22]. For Ga-BaY(IS), the intensity for the IR bands showed 67% Lewis and 33%

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and gave absorption bands at 1574 and 1595 cm−1, which confirmed the presence of weak and medium BrønstedBrønsted acid acid sites sites. for A theband 250 at 1489◦C calcined cm−1 was sample,due to the 73% contribution Lewis and of both 27% Lewis Brønsted and Brønsted for the 550 acid◦ sitesC calcined sample,[22]. For and Ga-BaY(IS), 77% Lewis the intensity and 23% for Brønstedthe IR bands sites showed for the67% 750Lewis◦C and calcined 33% Brønsted sample. acid Forsites Ga-BaY(Sil),for the at 250250 ◦°CC, calcined the band sample, intensities 73% Lewis implied and 27% 71% Brønsted Lewis for and the 29% 550 Brønsted°C calcined acid sample, sites, and at 77% 550 Lewis◦C, 74% and Lewis 23% Brønsted sites for the 750 °C calcined sample. For Ga-BaY(Sil), at 250 °C, the band intensities implied and 26% Brønsted acid sites, while the 750 ◦C calcined sample showed 77% Lewis and 23% for Brønsted acid71% sites. Lewis These and 29% results Brønsted suggest acid sites, that at upon 550 °C, increasing 74% Lewis the and calcination 26% Brønsted temperature, acid sites, while the the migration 750 °C of calcined sample showed 77% Lewis and 23% for Brønsted acid sites. These results suggest that upon framework metal (Al/Ga) to the extra framework also increases, which creates more Lewis acid sites increasing the calcination temperature, the migration of framework metal (Al/Ga) to the extra framework whilealso decreasing increases, which Brønsted creates acid more sites. Lewis acid sites while decreasing Brønsted acid sites.

C Absorbance (a.u) (a.u) Absorbance

1700.0 1680 1660 1640 1620 1600 1580 1560 1540 1520 1500 1480 1460 1440 1420 1400.0

Wavenumber (cm-1)

Catalysts 2020, 10, x FOR PEER REVIEW 6 of 13 FigureFigure 4. Pyridine-IR 4. Pyridine-IR spectra spectra ofof Ga-BaY(IS) calcined calcined at (A at) 250 (A) °C, 250 (B◦)C, 550 (B °C,) 550 (C) ◦750C, °C. (C ) 750 ◦C. Absorbance (a.u) (a.u) Absorbance

1700.0 1680 1660 1640 1620 1600 1580 1560 1540 1520 1500 1480 1460 1440 1420 1400.0

-1 Wavenumber (cm )

FigureFigure 5. Pyridine-IR 5. Pyridine-IR spectra spectra of Ga-BaY(Sil)of Ga-BaY(Sil) calcinedcalcined at at (A (A) 250) 250 °C,◦ (C,B) (550B) °C, 550 (C◦)C, 750 (C °C.) 750 ◦C.

2.1.5. Temperature-Programmed Desorption with NH3 Ammonia-temperature-programmed desorption (TPD) analysis was only performed on the catalysts that were calcined at 550 °C and 750 °C. Catalysts calcined at 250 °C were not analyzed as they might still contain some volatile organics that can damage the instrument when run under the conditions used for the ammonia-TPD analysis. The results in Table 1 show results in three ammonia desorption regions, which correspond to weak acid sites (≤200 °C), medium strength acid sites (<400 °C), and strong acid sites (≥400 °C) [23]. Comparing the results between the two catalysts calcined at 550 °C and 750 °C, one sees an increase in the concentration of medium strength and strong acid sites and a decrease in the weak acid sites when the catalysts were calcined at 750 °C. This suggests that calcining at higher temperatures leads to the dehydroxylation of the weakly bound extra framework hydroxyl groups responsible for the weak Brønsted acid sites, leaving behind bare cations acting as Lewis acid sites. The contribution of both framework cations and hydroxyl groups contained within the zeolitic material framework induced the medium strength Lewis and Brønsted acid sites. Strong acid sites, around NH3 desorption at 600 °C, are assigned to Lewis acid sites associated with bare cations of framework aluminum and gallium that migrate to the extra framework upon calcining at higher temperatures. Ga-BaY(IS) had more strong and total acid sites compared to Ga-BaY(Sil) because of the presence of aluminum in the Ga-BaY(IS) catalyst [24].

Table 1. Acid properties of zeolites calcined at different temperatures studied by NH3-temperature- programmed desorption (TPD).

Acidity Amount (µmol g−1) Catalyst W (100–200 °C) M (200–400 °C) S (400–600 °C) Total Acidity Ga-BaY(IS)-550 2.5 0.6 2.4 5.5 Ga-BaY(IS)-750 0.7 1.5 3.8 6.0 Ga-BaY(Sil)-550 1.0 0 1.1 2.1 Ga-BaY(Sil)-750 0.7 1.1 1.6 3.4

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2.1.5. Temperature-Programmed Desorption with NH3 Ammonia-temperature-programmed desorption (TPD) analysis was only performed on the catalysts that were calcined at 550 ◦C and 750 ◦C. Catalysts calcined at 250 ◦C were not analyzed as they might still contain some volatile organics that can damage the instrument when run under the conditions used for the ammonia-TPD analysis. The results in Table1 show results in three ammonia desorption regions, which correspond to weak acid sites ( 200 C), medium strength acid ≤ ◦ sites (<400 C), and strong acid sites ( 400 C) [23]. Comparing the results between the two catalysts ◦ ≥ ◦ calcined at 550 ◦C and 750 ◦C, one sees an increase in the concentration of medium strength and strong acid sites and a decrease in the weak acid sites when the catalysts were calcined at 750 ◦C. This suggests that calcining at higher temperatures leads to the dehydroxylation of the weakly bound extra framework hydroxyl groups responsible for the weak Brønsted acid sites, leaving behind bare cations acting as Lewis acid sites. The contribution of both framework cations and hydroxyl groups contained within the zeolitic material framework induced the medium strength Lewis and Brønsted acid sites. Strong acid sites, around NH3 desorption at 600 ◦C, are assigned to Lewis acid sites associated with bare cations of framework aluminum and gallium that migrate to the extra framework upon calcining at higher temperatures. Ga-BaY(IS) had more strong and total acid sites compared to Ga-BaY(Sil) because of the presence of aluminum in the Ga-BaY(IS) catalyst [24].

Table 1. Acid properties of zeolites calcined at diﬀerent temperatures studied by

NH3-temperature-programmed desorption (TPD).

1 Acidity Amount (µmol g− )

Catalyst W (100–200 ◦C) M (200–400 ◦C) S (400–600 ◦C) Total Acidity Ga-BaY(IS)-550 2.5 0.6 2.4 5.5 Ga-BaY(IS)-750 0.7 1.5 3.8 6.0 Ga-BaY(Sil)-550 1.0 0 1.1 2.1 Ga-BaY(Sil)-750 0.7 1.1 1.6 3.4

2.1.6. Thermogravimetric Analysis Thermogravimetric analysis of the prepared catalysts (Figure6) showed three events, in the temperature ranges 25–300 ◦C, 300–700 ◦C, and 700–1000 ◦C. Ga-BaY(Sil) showed distinct diﬀerences compared to Ga-BaY(IS). Ga-BaY(IS) calcined at 250 ◦C showed a total weight loss of up to 24% compared to about 21% total weight loss by Ga-BaY(Sil). Weight loss for the 550 ◦C and 750 ◦C calcined Ga-BaY(IS) was about 8% and 4%, respectively, compared to 15% and 10% of Ga-BaY(Sil). The weight loss at the low temperature range was due to the loss of water and hydroxyl groups weakly bound to the extra framework cations. The weight loss at intermediate temperature ranges was due dehydroxylation within the framework of the zeolitic material [25]. This was more apparent for Ga-BaY(IS) which contains more of these sites, conﬁrmed by the higher concentration and strength of Brønsted acid sites (Section 2.1.5). Ga-BaY(Sil) was unstable at very high temperatures, shown by a sharp decrease in the thermogravimetric analysis (TGA) curve just below 1000 ◦C. Ga-BaY(IS) was stable up to 1000 ◦C, suggesting that the complete substitution of aluminum with gallium in the framework of the zeolitic material decreases thermal stability. Catalysts 2020, 10, x FOR PEER REVIEW 7 of 13

2.1.6. Thermogravimetric Analysis Thermogravimetric analysis of the prepared catalysts (Figure 6) showed three events, in the temperature ranges 25–300 °C, 300–700 °C, and 700–1000 °C. Ga-BaY(Sil) showed distinct differences compared to Ga-BaY(IS). Ga-BaY(IS) calcined at 250 °C showed a total weight loss of up to 24% compared to about 21% total weight loss by Ga-BaY(Sil). Weight loss for the 550 °C and 750 °C calcined Ga-BaY(IS) was about 8% and 4%, respectively, compared to 15% and 10% of Ga-BaY(Sil). The weight loss at the low temperature range was due to the loss of water and hydroxyl groups weakly bound to the extra framework cations. The weight loss at intermediate temperature ranges was due dehydroxylation within the framework of the zeolitic material [25]. This was more apparent for Ga-BaY(IS) which contains more of these sites, confirmed by the higher concentration and strength of Brønsted acid sites (Section 2.1.5). Ga- BaY(Sil) was unstable at very high temperatures, shown by a sharp decrease in the thermogravimetric analysis (TGA) curve just below 1000 °C. Ga-BaY(IS) was stable up to 1000 °C, suggesting that the complete Catalysts 2020, 10, 975 7 of 13 substitution of aluminum with gallium in the framework of the zeolitic material decreases thermal stability.

Ga-BaY(IS)-250oC 100 100 Ga-BaY(IS)-550oC Ga-BaY(Sil)-250oC Ga-BaY(IS)-750oC 95 Ga-BaY(Sil)-550oC 95 Ga-BaY(Sil)-750oC 90 90

85 85 Weight Loss/% Weight Loss/ % 80 80

75 75 70 200 400 600 800 1000 200 400 600 800 1000 o Temparature/ C Temperature/ oC (A) (B)

FigureFigure 6. Thermogravimetric 6. Thermogravimetric anal analysisysis curves curves of (A of) Ga-BaY(Sil) (A) Ga-BaY(Sil) and and(B) Ga-BaY(IS) (B) Ga-BaY(IS) calcined calcined at different at temperatures.diﬀerent temperatures.

2.1.7. Brunauer, Emmett, and Teller (BET) Studies 2.1.7. Brunauer, Emmett, and Teller (BET) Studies BET surface studies were carried out using the catalysts calcined at 250 C and 750 C to evaluate BET surface studies were carried out using the catalysts calcined at 250 °C◦ and 750 ◦°C to evaluate the the eﬀect of calcination temperature on the surface area and pore volumes of the prepared catalysts effect of calcination temperature on the surface area and pore volumes of the prepared catalysts (Table 2). (Table2). For both the catalysts (Ga-BaY(IS) and Ga-BaY(Sil)), surface areas decreased with increasing For both the catalysts (Ga-BaY(IS) and Ga-BaY(Sil)), surface areas decreased with increasing calcination temperature.calcination For temperature. the pore volumes, For the pore only volumes, a slight chan onlyge a slightwas observed, change was which observed, also shows which a alsodecrease shows in apore volumedecrease as ina result pore volume of partial as apore result blockage of partial resulting pore blockage from resultingcation migration from cation within migration the zeolitic within material the framework.zeolitic material There framework.was also a slight There wasincrease also ain slight the pore increase diameter in the as pore the diameter calcination as the temperature calcination was increased.temperature was increased.

Table 2. Surface properties of Ga-BaY(IS) and Ga-BaY(Sil) calcined at diﬀerent temperatures. Table 2. Surface properties of Ga-BaY(IS) and Ga-BaY(Sil) calcined at different temperatures. 2 3 CatalystsCatalysts BET (m 2/g) Pore Pore Volume Volume (cm (cm3/g)/ g) PorePore Size Size (nm) (nm) Ga-BaY(IS)-250Ga-BaY(IS)-250 734 0.33 0.33 17 17 Ga-BaY(IS)-750Ga-BaY(IS)-750 709 0.32 0.32 18 18 Ga-BaY(Sil)-250 492 0.22 18 Ga-BaY(Sil)-250 492 0.22 18 Ga-BaY(Sil)-750 472 0.21 19 Ga-BaY(Sil)-750 472 0.21 19

N2 adsorption-desorption for the characterized catalysts all showed type IV isotherms (Figures S3–S6) which isN 2typicaladsorption-desorption of mesoporous solids. for The the hysteresis characterized loops catalysts for the Ga-BaY(IS) all showed catalysts type IVwere isotherms classified as H4(Figures hysteresis S3–S6) loops, which which is suggests typical ofnarrow mesoporous slit-like pores, solids. with The walls hysteresis composed loops of formesoporous the Ga-BaY(IS) silica. Ga- BaY(Sil)catalysts shows were H2 classiﬁed type hysteresis as H4 loops hysteresis which loops, are indi whichcative suggests of a material narrow with slit-like a narrow pores, mouth with to wallsthe pores composed of mesoporous silica. Ga-BaY(Sil) shows H2 type hysteresis loops which are indicative of a material with a narrow mouth to the pores and uniform channel-like pores [26]. The increase in adsorbed nitrogen volume at high relative N2 pressures for all the prepared samples relates to the presence of the intercrystalline porosity [27].

2.2. Catalytic Performance Catalytic testing results shown in Figure7 were obtained under similar testing conditions 1 (Temperature = 450 ◦C, GHSV = 6000 h− , C:O ratio = 8:1, and catalyst = 1 mL). Ga-BaY(Sil) calcined at diﬀerent temperatures showed higher conversions when compared to Ga-BaY(IS) calcined under similar conditions. The higher conversions over Ga-BaY(Sil) catalysts may be attributed to the higher loading of Ga compared to the Ga-BaY(IS) catalysts, namely, three magnitudes higher. Conversion over both the catalysts decreased with increasing calcination temperature, and this suggests that as the calcination temperature increases, dehydroxylation of the catalysts leads to the decrease in the concentration of hydroxyl groups responsible for the Brønsted acid sites, which in turn decreases Catalysts 2020, 10, x FOR PEER REVIEW 8 of 13

and uniform channel-like pores [26]. The increase in adsorbed nitrogen volume at high relative N2 pressures for all the prepared samples relates to the presence of the intercrystalline porosity [27].

2.2. Catalytic Performance Catalytic testing results shown in Figure 7 were obtained under similar testing conditions (Temperature = 450 °C, GHSV = 6000 h−1, C:O ratio = 8:1, and catalyst = 1 mL). Ga-BaY(Sil) calcined at different temperatures showed higher conversions when compared to Ga-BaY(IS) calcined under similar Catalystsconditions.2020, 10, 975The higher conversions over Ga-BaY(Sil) catalysts may be attributed to the higher loading of Ga8 of 13 compared to the Ga-BaY(IS) catalysts, namely, three magnitudes higher. Conversion over both the catalysts decreased with increasing calcination temperature, and this suggests that as the calcination temperature the activityincreases, of dehydroxylation the catalysts. Theof the selectivity catalysts leads to cracked to the decrease products, in the which concentration are a result of hydroxyl of the groups acidity of the catalyst,responsible also for decreases the Brønsted with acid the sites, calcination which in turn temperature. decreases the Thisactivity also of the suggests catalysts. the The decrease selectivity in the concentrationto cracked ofproducts, the Brønsted which are acid a result sites asof the acidity calcination of the temperaturecatalyst, also decreases is increased. with Thethe calcination low cracking temperature. This also suggests the decrease in the concentration of the Brønsted acid sites as the calcination activity shown by catalysts calcined at 750 C results from the induced Lewis acid sites from the temperature is increased. The low cracking activity◦ shown by catalysts calcined at 750 °C results from the dehydroxylationinduced Lewis and acid migration sites fromof the the dehydroxylation framework cations. and migration This is of supported the framework by the cations. results This shown is in Tablesupported3. by the results shown in Table 3.

Ga-BaY(IS) Ga-BaY(Sil) Conversion(IS) Conversion(Sil)

30 9 8 25 7 20 6 5 15 4 10 3 % conversion 2 5 1 0 0 %Cracked Products Selectivity Products %Cracked 250 550 750 o Calcination Temperature/ C . Figure 7. Conversion and cracked products selectivity of Ga-BaY(IS) and Ga-BaY(Sil) calcined at Figure 7. Conversion and cracked products selectivity of Ga-BaY(IS) and Ga-BaY(Sil) calcined at different diﬀerenttemperatures. temperatures. Table 3. Cracked products distribution over Ga-BaY(IS) and Ga-BaY(Sil) calcined at different temperatures. Table 3. Cracked products distribution over Ga-BaY(IS) and Ga-BaY(Sil) calcined at different temperatures.

CatalystsCatalysts %Alkanes %Alkanes %Alkenes %Alkenes Ga-BaY(IS)-250Ga-BaY(IS)-250 11 1189 89 Ga-BaY(IS)-550Ga-BaY(IS)-550 3.0 3.097 97

Ga-BaY(IS)-750Ga-BaY(IS)-750 100100 Ga-BaY(Sil)-250Ga-BaY(Sil)-250 6 694 94 Ga-BaY(Sil)-550Ga-BaY(Sil)-550 1.0 1.099 99 Ga-BaY(Sil)-750Ga-BaY(Sil)-750 100100

Table3 shows the product distribution of the cracked products from the tested catalysts. Table 3 shows the product distribution of the cracked products from the tested catalysts. The catalysts The catalysts calcined at 250 C showed both cracked alkanes and alkenes in the products stream, calcined at 250 °C showed ◦both cracked alkanes and alkenes in the products stream, whereas as the whereascalcination as the calcinationtemperature temperaturewas increased, was the increased, alkane-cracked the alkane-cracked products were productsnot within were detectable not within detectableconcentrations. concentrations. However, However,for Ga-BaY(IS), for the Ga-BaY(IS), disappearance the of disappearance alkane-cracked products of alkane-cracked was only observed products was onlyfor the observed 750 °C calcined for the catalyst. 750 ◦C This calcined was attributed catalyst. to the This higher was concentration attributed to of the strong higher Brønsted concentration acid sites of strongassociated Brønsted with acid the sites Ga-BaY(IS) associated catalyst with compared the Ga-BaY(IS) to Ga-BaY(Sil). catalyst compared to Ga-BaY(Sil). The formation of cracked products follows diﬀerent mechanisms. For the catalysts calcined at 250 ◦C, the higher concentration of Brønsted acid sites may activate the feed molecule (n-octane) via protonation and formation of the carbonium ion. When this carbonium ion cracks, it yields an alkane and an adsorbed carbenium ion, which also cracks via ß-scission to yield an alkene and a smaller carbenium ion. This mechanism is more pronounced and faster, which explains the higher conversions at low temperature calcined catalysts [28]. As the catalysts dehydroxylate at higher calcination temperatures, Brønsted acid sites decrease, and Lewis acid sites increase. The disappearance of alkane-cracked products is characteristic of a Lewis acid activated reaction. Lewis acid sites generate a carbenium ion directly from the feed molecule via abstraction of a hydride ion, which is followed by cracking via repeated ß-scissions. This mechanism leads to the formation of alkene-cracked products and it is slower, which explains the lower conversions for higher temperature calcined catalysts. Only C3–C5 cracked products were observed under testing conditions. C1–C2 may have been produced in small amounts and were unstable since primary carbonium ions require high energy to form. Figures8 and9 show octenes selectivity and CO x selectivity, respectively. Less acidic Ga-BaY(Sil) showed higher octene selectivity, up to 36% for the 750 ◦C calcined catalyst compared to 27% for Ga-BaY(IS) catalyst. For both the catalysts, octene selectivity increased with increasing calcination Catalysts 2020,, 10,, xx FORFOR PEERPEER REVIEWREVIEW 9 of 13

The formation of cracked products follows different mechanisms. For the catalysts calcined at 250 °C, thethe higherhigher concentrationconcentration ofof BrønstedBrønsted acidacid sitessites maymay activateactivate thethe feedfeed moleculemolecule ((n-octane) via protonation and formation of the carbonium ion. When this carboniumium ionion cracks,cracks, itit yieldsyields anan alkanealkane andand anan adsorbedadsorbed carbenium ion, which also cracks via ß-scission to yield an alkene and a smaller carbenium ion. This mechanism is more pronounced and faster, which explains the higher conversions at low temperature calcined catalysts [28]. As the catalysts dehydroxylate at higher calcination temperatures, Brønsted acid sites decrease, and Lewis acid sites increase. The disappearance of alkane-cracked products is characteristic of a Lewis acid activated reaction. Lewis acid sites generate a carbenium ion directly from the feed molecule via abstraction of a hydride ion, which is followed by cracking via repeated ß-scissions. This mechanism leadsleads toto thethe formationformation ofof alkene-crackedalkene-cracked productsproducts andand itit isis slower,slower, whichwhich explainsexplains thethe lowerlower conversionsconversions forfor higher temperature calcined catalysts. Only C33–C55 crackedcracked productsproducts werewere observedobserved underunder testingtesting conditions. C11–C22 maymay havehave beenbeen producedproduced inin smallsmall amountsamounts andand werewere unstableunstable sincesince primaryprimary carboniumcarbonium ionsions requirerequire highhigh energyenergy toto form.form.

CatalystsFigure2020 8 and, 10, Figure 975 9 show octenes selectivity and COx selectivity,selectivity, respectively.respectively. LessLess acidicacidic Ga-BaY(Sil)Ga-BaY(Sil)9 of 13 showed higher octene selectivity, up to 36% for the 750 °C°C calcinedcalcined catalystcatalyst comparedcompared toto 27%27% forfor Ga-BaY(IS)Ga-BaY(IS) catalyst. For both the catalysts, octene selectivity increased with increasing calcination temperature, which temperature, which suggests the decrease in the concentration of Brønsted acid sites which lead to suggests the decrease in the concentration of Brønsted acid sites which lead to deep oxidation of the octene deep oxidation of the octene products. For COx, in Figure9, a decrease is seen from the 250 C calcined products. For COx,, inin FigureFigure 9,9, aa decreasedecrease isis seenseen fromfrom thethe 250250 °C°C calcinedcalcined catalystscatalysts toto thethe 550550◦ °C°C calcinedcalcined catalysts to the 550 C calcined catalysts, which suggested the decrease in Brønsted acid sites which catalysts, which suggested◦ the decrease in Brønsted acid sites which lead to over-oxidation to COx.. TheThe lead to over-oxidation to COx. The increase in COx from the 550 C to 750 C calcined catalysts may increaseincrease inin COCOx fromfrom thethe 550550 °C°C toto 750750 °C°C calcinedcalcined catalystscatalysts maymay bebe◦ attributedattributed◦ toto thethe formationformation ofof moremore be attributed to the formation of more strong Lewis acid sites at this temperature, which leads to the strong Lewis acid sites at this temperature, which leads to the strong adsorption of alkene products, and strong adsorption of alkene products, and subsequently, leads to deep oxidation to COx. subsequently, leads to deep oxidation to COx..

Ga-BaY(IS) Ga-BaY(Sil) 40 35 30 25 20 15 10

%Octenes Selectivity 5

%Octenes Selectivity 5 0 200 300 400 500 600 700 800 Temp./ ooC

Figure 8. OcteneOctene selectivity selectivity of of Ga-BaY(IS) Ga-BaY(IS) and Ga-BaY Ga-BaY(Sil)(Sil)(Sil) calcined calcined at at different didifferentﬀerent temperatures.temperatures.

60 GaBaY(IS) GaBaY(Sil) 50

20 % COx Selectivity% COx % COx Selectivity% COx 10

0 200 300 400 500 600 700 800 Temp./ ooC

Figure 9. COx selectivity of Ga-BaY(IS) and Ga-BaY(Sil) calcined at diﬀerent temperatures.

Iso-conversion data (Figures S7–S10) of the prepared catalysts showed similar trends, which suggests insigniﬁcant conversion eﬀects on the product selectivities obtained when the catalysts were tested under similar conditions. The catalysts were stable and, e.g., Figure S11 shows no coke formation over BaY(IS) after a three-day catalytic run.

3. Materials and Experimental Methods

3.1. Catalysts Preparation Both studied catalysts were prepared by isomorphic substitution using a modiﬁed sol-gel method [29]. Synthesis of Ga-BaY(IS) was carried out in a 500 mL sealable Teﬂon beaker, where 14.53 g of BaCl 2H O (Merck NT Laboratory Suppliers, Kenilworth, NJ, USA) was dissolved in 37.5 g of 2· 2 distilled water, followed by the addition of 6.56 g of aluminum tri-sec-butoxide (Merck NT Laboratory Suppliers, Kenilworth, NJ, USA)) and 0.74 g of gallium nitrate (Sigma Aldrich, St. Louis, MO, USA) under agitation for 2 h to yield gallium aluminate solution. After 2 h, 30 wt% colloidal silica solution (Sigma Aldrich—26.44 g, St. Louis, MO, USA) was slowly poured to the aluminate solution under Catalysts 2020, 10, 975 10 of 13

vigorous stirring. The beaker was then tightly sealed and transferred to an oil bath at 25 ◦C and aged for 24 h with constant stirring. Thereafter, the solution was aged at 40 ◦C for 24 h with no stirring, and finally at 80 ◦C for 48 h. The recovered solid was filtered under vacuum and washed with double distilled water until the pH was 8,9 and dried at 110 ◦C overnight. For the synthesis of Ga-BaY(Sil), 3.023 g of gallium nitrate was used in place of aluminum tri-sec-butoxide, but all other conditions and procedures were kept similar to the synthesis of Ga-BaY(IS). The coding used to distinguish between the two prepared catalysts is IS (for isomorphically substituted zeolite) and Sil (for the silicalite). After structure confirmation characterization (XRD, SEM, and FTIR) was carried out on the prepared catalysts, each sample was separated into three batches. One batch was calcined under flowing air at 250 ◦C for six hours, the next batch at 550 ◦C, and the last batch at 750 ◦C before catalytic testing.

3.2. Catalysts Characterization Powder X-ray diffraction was carried out for structure and phase identification using a Bruker D8 Advance diffractometer, with a graphite monochromatic filter operated at 40 kV and 40 mA (Karlsruhe, Germany), and CuKα radiation (λ = 1.5406 nm). Data was collected in steps of 0.02◦ 1 and scanning speed of 0.2 s− , in the 2-theta range of 5◦–90◦.N2 physisorption was carried out with a Micromeritics Tristar II surface area and porosity analyzer at 196 C (Norcross, GA, USA). − ◦ Before analysis, finely ground samples were degassed up to their calcination temperature under N2 flow overnight in a Micromeritics Flow Prep 060 (Norcross, GA, USA). The catalysts calcined at 750 ◦C were degassed at 550 ◦C because the degassing instrument was limited to that temperature. Temperature-programmed experiments were carried out on a Micromeritics 2920 Autochem II Chemisorption Analyser (Norcross, GA, USA). Ammonia temperature-programmed desorption (NH3-TPD) was carried out with ca. 0.06 g catalyst following a reported method [30]. Infrared spectra were obtained at room temperature with a Perkin Elmer Spectrum 100 FT-IR Spectrometer equipped with a Universal ATR Sampling Accessory (Waltham, MA, USA). The pyridine IR spectra were obtained with the same instrument as above. For the pyridine IR, 0.05 g samples were treated with pyridine (1.0 mL) and dried under flowing air at room temperature. Then, the spectra were recorded in a range 1 of 1400–1700 cm− . A Zeiss Ultra Plus field emission gun scanning electron microscope (FEG-SEM) with Smart SEM Software (Oberkochen, Germany) was used for obtaining SEM and SEM-EDX images. Before the analyses, the samples were coated with gold using a Q150R series high vacuum Quorum sputter coater (Laughton, UK). Thermogravimetric analyses (TGA) were carried out with a Perkin Elmer STA 6000 (Waltham, MA, USA). The samples were subjected to a temperature ramp from 25 ◦C to 1000 ◦C in air.

3.3. Catalytic Testing Catalytic testing was carried out using a laboratory scale continuous-flow, gas phase, fixed-bed reactor at 450 ◦C. Air and N2 were fed to the reactor as the oxidant and diluent gas, respectively. n-Octane (Merk, NJ, USA) with a purity of >98% was fed at a concentration (v/v) in the gaseous mixture above n-octane’s upper flammability limit using a calibrated Lab Alliance Series II HPLC Pump (New York, NY, USA). The mass delivered was recorded using an electronic balance. The n-octane fed was kept in the gaseous phase by heated feed lines at 130 ◦C using heating tape. Temperatures were controlled with CB-100 RK temperature control units with internal relays and monitored via K-type thermocouples. The pelletized catalysts (1.0 mL) were placed between two thin layers of glass wool and positioned in the hottest zone of the calibrated reactor block. The spaces in the reactor tube were packed with 24 gritt carborundum and stoppered by glass-wool at either end. A Ritter wet gas flow meter measured the total gas flow. The liquid products and unreacted feed were collected in a cylindrical stainless steel catch vessel at ca. 3.0 ◦C. The gaseous products were analyzed using a Perkin Elmer Clarus 400 gas chromatograph (GC) (Waltham, MA, USA) fitted with a 30 m 530 µm Supelco × Carboxen 106 PLOT column and equipped with a thermal conductivity detector (TCD). The liquid and gaseous products from the cooled catch vessel were analyzed using a Shimadzu GC-2025 (Kyoto, Japan), Catalysts 2020, 10, 975 11 of 13

ﬁtted with a 50 m 200 µm PONA capillary column and ﬂame ionization detector (FID). To obtain × 1 1 iso-conversion, the GHSV was varied in the range of 6000 h− for the less active catalysts to 8000 h− for the more active catalysts.

4. Conclusions Thermal treatments of the zeolitic materials dehydroxylate the materials leading to a decrease in the bridging hydroxyl groups responsible for the Brønsted acidity of the material. At temperatures ranging from 550 ◦C and above, there is a partial and full migration of framework and extra framework cations in the zeolitic material, which induces Lewis acid sites of diﬀerent acid strengths. Thermally treated zeolitic materials activate n-octane following two mechanistic routes, namely, a monomolecular mechanism and bimolecular mechanism. The monomolecular mechanism is Brønsted acid site facilitated and bimolecular mechanism is facilitated by Lewis acid sites. The products’ selectivity showed that for low temperature (250 ◦C) calcined catalysts, the dominating mechanism is the monomolecular mechanism. For high temperature (750 ◦C) calcined catalysts, the dominating mechanism was found to be the bimolecular mechanism.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/10/9/975/s1, Scheme S1: Modification of zeolites by isomorphic substitution to yield a silicalite material, Scheme S2: Transformation of acid sites of zeolitic materials, Figure S1: XRD diffractograms of (A) Ga-BaY(IS), (B) Ga-BaY(Sil), Figure S2: FT-IR spectra of (A) Ga-BaY(IS), (B) Ga-BaY(Sil), Figure S3: N2 adsorption-desorption isotherms of Ga-BaY(Sil) calcined at 250 ◦C, Figure S4: N2 adsorption-desorption isotherms of Ga-BaY(Sil) calcined at 750 ◦C, Figure S5: N2 adsorption-desorption isotherms of Ga-BaY(IS) calcined at 250 ◦C, Figure S6: N2 adsorption-desorption isotherms of Ga-BaY(IS) calcined at 750 ◦C, Figure S7: Cracked products selectivity at iso-conversion for Ga-BaY(IS) and Ga-BaY(Sil) calcined at different temperatures (6 1 % and 8 1 % respectively), Figure S8: Cracked products distribution at iso-conversion for Ga-BaY(IS) and Ga-BaY(Sil)± calcined± at different temperatures, Figure S9: Octene selectivity at iso-conversion of Ga-BaY(IS) and Ga-BaY(Sil) calcined at different temperatures, Figure S10: COx selectivity at iso-conversion of Ga-BaY(IS) and Ga-BaY(Sil) calcined at different temperatures, Figure S11: Images of (A) used BaY (Before Ga substitution) with coke deposits, and (B) used Ga-BaY(IS) free of coke deposits after a 3 day run. Author Contributions: Synthesis, characterization, catalysis measurements: S.S.N.; data analysis: S.S.N., H.B.F. and M.N.C.; project supervision and manuscript preparation: H.B.F. and M.N.C. All authors read and agreed to the published version of the manuscript. Funding: This research was funded by C*Change, the DST-NRF Centre of Excellence in catalysis and SASOL. Acknowledgments: We would like to thank the School of Chemistry and Physics, Microscopy and Microanalysis Unit (UKZN) for characterization and M. Datt (Sasol) for constructive criticism for this research. Conflicts of Interest: The authors declare no conflict of interest.

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